Lecture Notes: Evolution of Amniotic Eggs

Introduction

• Walruses, rattlesnakes, and parakeets share a common evolutionary ancestry.
• 350 million years ago: Dominant land vertebrates were amphibians.
• Amphibian characteristics:
  • Range from newts to crocodiles.
  • Require water for egg-laying due to their shell-less, jelly-like eggs.

Evolutionary Breakthrough: The Amniotic Egg

• Amniotic Egg: A shelled, waterproof egg allowing laying on dry land.
• Amniotes: First tetrapods resembling small lizards capable of terrestrial life.
• Amniotic egg allows avoidance of water for egg-laying.
• Brings aquatic environment within the egg's shell.

Key Developments of Amniotic Eggs

1. Protective Shell

• Tough, flexible, and leathery (similar to some modern reptile eggs).
• Functions:
  • Protects from predators, bacteria, damage, and dehydration.
  • Porous structure allows oxygen exchange for embryo.

2. Chorion

• First membrane: Protective layer & passage for oxygen/carbon dioxide.
• Recognizable as the thin skin of hard-boiled eggs.
• Acts as in and out doors for gases.

3. Allantois

• Works with chorion to manage gas exchange.
• Stores waste from the embryo.
• Acts as a lobby directing oxygen and carbon dioxide.

4. Amnion

• Membrane holding fluid for the embryo's floating environment.
• Prevents embryo's dehydration; serves as a shock absorber.
• Key in allowing terrestrial egg-laying.

Evolutionary Significance

• Creates a safe, watery environment for embryo development.
• Amniotes split into two groups:
  • Synapsida: Ancestors of mammals (e.g., walruses).
  • Sauropsida: Ancestors of reptiles, birds, and dinosaurs (e.g., rattlesnakes, parakeets).
• Represents a family reunion of diverse species sharing evolutionary roots in the amniotic egg.